In modern electronic organs and other electronic tone producing devices of the key actuated type, it has been the practice to employ circuit arrangements for modifying the decay times of tones upon release of a key. This is done to produce percussion effects or to produce natural sounding tonal effects representative of other instruments. Percussion keyers are becoming increasingly popular in conjunction with electronic organs which include circuitry for automatically or semiautomatically producing a percussion accompaniment to the playing of melody and chords on the manuals of the organ. The percussion accompaniment generally is controlled by a rhythm generator system which can be switched into and out of operation at the will of the musician playing the organ. Different rhythm patterns for producing different types of percussive effects are employed.
In the past, it has been common to utilize resistive-capacitive (RC) networks for establishing the desired time delays to create the tone amplitude decay rate used to create percussion effects. The capacitors in such timing circuits have relatively large values of capacitance, and it has been necessary to use large numbers of such RC circuits corresponding to the number of keyers used in the instrument. Variations in tolerances of such capacitors make it difficult to consistently reproduce the same effects from one instrument to another. Similar difficulty also is encountered within the same instrument in obtaining uniform decay characteristics among the various keys used in the instrument when RC delay circuits are used.
Another disadvantage of the use of RC timing circuits in electroinc musical instruments is that, at the present state of the art, it is not economically feasible to form large capacitors as part of integrated circuits. Thus, as integrated circuitry is used for other portions of the logic function of an electronic instrument, it still is necessary to employ discrete capacitors. This requires additional bonding pads on the integrated circuit chips, reducing the useable chip area, and also increases manufacturing costs because of the hybrid nature of the circuitry.
The above identified U.S. Pat. No. 3,977,291 discloses an attenuator network which may be used to produce percussion effects or other time delays in the attack and decay of a keyer used in conjunction with an attenuator which does not employ RC timing circuits. Instead, a switching network controlled by a counter is used to vary the attenuation of the tone signals applied to the attenuator network according to a predetermined set pattern. Such a keyer-attenuator network is ideally suited for use in a percussion system in conjunction with a rhythm generator to produce the desired percussion sounds or tones. A disadvantage, however, exists in the use of this circuit as a percussion keyer inasmuch as it is possible to switch on the attenuator (representative of the inception of a tone) at any time during the cycle of an applied sine wave signal to the input of the attenuator. If the attenuator/keyer network is turned on when the input tone signal has a maximum amplitude (as is the case for percussion sounds), very unpleasant sound effects are produced at the start of the tone. Such tone effects are unnatural and tend to detract from the overall musical quality of any selection which is being played when such an effect is produced. Since the oscillators of the tone keyers are free running and the attenuators may be switched to pass tones from the tone oscillators to their outputs at any random time in the cycle of the tone waveform, even the unpleasant tone effects are not consistent. Therefore, it is difficult to take countermeasures to effectively eliminate these effects. Such effects are most noticeable in conjunction with digital percussion keyers since the attack of such tones is practically instantaneous to maximum tone output (minimum attenuation) with a repidly decaying amplitude after the initial attack.
Therefore, it is desirable to provide a digital keyer, and particularly a digital percussion keyer, which has a realistic sound and which consistently switches the keyer on and varies the tone attenuation at the zero crossings of the sine wave signals produced by the tone generator used to supply input signals to the attenuator.